This invention pertains to receivers, and more specifically to a novel RF receiver which is capable of being remotely tuned in frequency and bandwidth.
The need for radio receivers is widespread, and a recent use which has experienced phenomenal growth is the cellular telephone. Cellular telephones have undergone a dramatic market growth in the past few years. These existing systems utilize analog FM modulation techniques. In order to transmit data, landline modem signals are transmitted over cellular systems by using the cellular telephone as a twisted pair replacement. The trend in the industry is toward replacing the analog FM system with digital modulation and transmission means, e.g. GSM, IS54, JDC. IS54 is a so-called dual mode system in which the existing analog and the new digital modulation must coexist. Thus one portable handset must be capable of communicating using either analog or digital cellular signals. Other types of signals are being transmitted over the cellular network, as described in U.S. Pat. No. 4,914,651. This diversity of signals causes the designer of terminal equipment to require a receiver which is capable of dealing with multiple modulation techniques, or "protocols."
FIG. 1 shows a typical prior art double conversion receiver architecture. Double conversion receiver 100 receives an RF input signal on antenna 101 which is applied to RF section 102. RF section 102 includes RF filter 103 (such as a Surface Acoustic Wave filter) which provides to low noise amplifier 104 an RF signal with a desired passband and including the desired signal to be received having a carrier frequency .omega..sub.C. The output from low noise amplifier 104 is applied to automatic gain control circuit (AGC) 105 in order to provide an output signal to RF amplifier 106 of relatively constant amplitude independent of the amplitude of the RF signal received on antenna 101. AGC circuit 105 receives its control signal from any well-known means, for example an adaptive gain control loop implemented in the baseband portion of the receiver and for simplicity, not shown in FIG. 1. The output signal from RF amplifier 106 is applied to first mixer 107, which also receives a first local oscillator signal LO1 having a frequency .omega..sub.L01. The output signal from mixer 107 includes four primary frequency components: .omega..sub.C, .omega..sub.LO1, .omega..sub.C +.omega..sub.LO1, and .omega..sub.C -.omega..sub.LO1. The output from mixer 107 is applied to intermediate frequency (IF) stage 108 which includes amplifier 109 for amplifying the output signal from mixer 107. This amplified signal from amplifier 109 is applied to the bandpass filter 110 which is tuned to reject signals having frequencies .omega..sub.C and .omega..sub.LO1, as well as one of the remaining signal components of the output signal from mixer 107. This remaining component is the IF signal of interest and corresponds to the RF signal received on antenna 101 translated to a new intermediate frequency .omega..sub.IF. The output from band pass filter 110 is applied to the input of amplifier 111, whose output signal is applied to second mixer 112 for mixing with a second local oscillator signal LO2 having a frequency .omega..sub.LO2.
The output signal from mixer 112 is applied to baseband circuitry 113, which includes baseband amplifier 114 which is in turn coupled to baseband filter 115. The output signal from second mixer 112 includes frequency components .omega..sub.IF, .omega..sub.LO2, .omega..sub.IF +.omega..sub.LO2, and .omega..sub.IF -.omega..sub.LO2. Baseband filter 115 is tuned to reject frequencies .omega..sub.IF, .omega..sub.LO2, as well as one of the remaining frequency components output from mixer 112. The remaining frequency component of the output signal of mixer 112 is passed by passband filter 115 as the desired baseband signal containing quadrature components I and Q representing the information stored in the modulated RF signal received at antenna 101.
The optimum receiver architecture for a vector based communication channel like those of interest in personal communications is discussed by Wozencraft and Jacobs, "Principals of Communication Engineering," Chapter 4, P 211-285, John Wiley and Sons, 1965. The quadrature demodulator receiver shown in FIG. 1 is a common implementation of the design discussed in Wozencraft and Jacobs' text.
Ian Sevenhans, Amoul Vanwelsenaers, I. Wenin, J. Baro, "An Integrated Si bipolar RF transceiver for a zero IF 900 MHz GSM digital mobile radio frontend of a hand portable phone." IEEE 1991 Custom Integrated Circuits Conference, Paper 7.7, describes an implementation of a direct conversion receiver which is implemented using the traditional all analog approach.
FIG. 2 shows a prior art direct conversion receiver as described in "Performance of a Direct Conversion Receiver with pi/4-dqpsk Modulated Signal," K. Anvari, M. Kaube, and B. Hriskevich, Proceedings of the 41st IEEE Vehicular Technology Conference, May 1991, pp. 822-823. The major advantage of this type of receiver is that it allows for a reduction in both size and power consumption, as compared with a double conversion receiver as in FIG. 1. A modulated RF signal is received at antenna 201 and is applied to RF section 202 including RF amplifier 204. The amplified signal from amplifier 204 is applied to AGC circuit 205, which provides to the input of RF amplifier 206 a RF signal of relatively constant amplitude, independent of the amplitude of the received signal at antenna 201. As is well known in the art, AGC circuit 205 receives its control signal from any convenient source, such as an adaptive gain control loop implemented in the baseband portion of the receiver, and for simplicity not shown in FIG. 2.
The amplified RF signal from amplifier 206 is applied to quadrature demodulator 213. This output signal from amplifier 206 is applied to mixers 220I and 220Q, which each also receives a local oscillator signal having frequency .omega..sub.LO, but which are 90 degrees out of phase. Since the circuit of FIG. 2 is a direct conversion receiver, the local oscillator signal LO is tuned to the received signal frequency .omega..sub.C. Thus, mixer 220I receives from phase splitter 219 a signal -sin .omega..sub.c t and mixer 220Q receives from phase shifter 219 a signal cos .omega..sub.c t. Mixers 220I and 220Q provide baseband output signals, which are filtered of spurious signals by filter 221I and 221Q, amplified by amplifiers 222I and 222Q, and anti-alias filtered by filters 223I and 223Q, respectively. These I and Q baseband frequency components are applied from filters 223I and 223Q to analog to digital converter 224 (having a low pass characteristic) to provide digital I and Q baseband output signals providing the information contained in the modulated RF signal received at antenna 201.
Anvari, et al discuss various types of implementation problems with the direct conversion receiver. These are now reviewed for the purpose of showing how these problems are reduced or eliminated with the present invention.
1) Balance of Amplitude and Phase Terms
Distortion of the phasor results when the components of the I and Q channels are either uniformly distorted or differentially distorted. Anvari, et al. conclude that to avoid this type of distortion all the components in the I and Q channels require constant gain and phase characteristics across their dynamic range. To assure such constant gain and phase characteristics is difficult and costly.
2) Spurious Signal Rejection Filters
Sharp cutoff requirements for spurious signal rejection filters (such as filters 221I and 221Q of FIG. 2) cause amplitude and phase distortion for signals whose frequencies are close to the band edge. Sharp cutoff filters require a large number of filtering stages which increase complexity and cost. This is particularly true when such filters are implemented as switched capacitor filters which are difficult to implement, control, and test.
3) DC Offset in the I and Q Channels
DC offset is caused by carrier feedthrough from the high power local oscillator signals, self mixing of I and Q channels signals, 1/f noise of the operational amplifiers and mixers, and bias in the filters and amplifiers.
Anvari, et at. conclude that the problem can be eliminated by AC coupling of the signal or removing the DC offset in the digital signal processing section. Many of the modulation schemes used in personal communications require low frequency information to achieve low bit error rates. Thus predicting the DC offset in a control loop in the digital signal processor is the best prior art alternative, as this results in a lower bit error rate than is possible by simply AC coupling.
4) Sampling Time of the Analog to Digital Converter
The sampling jitter of the sample and hold used at the input to the Analog-to-Digital converter (such as Analog-to-Digital converter 224, FIG. 2) will introduce differential phase distortion between the I and Q channels.
The key elements of a receiver design are its physical size, power consumption, and cost. In the past, integration onto silicon of major portions of a portable terminal have accomplished these goals. The present invention allows for the further integration of the receiver as compared with the prior art. The present invention simplifies the construction of direct conversion receivers for a variety of personal communication systems.